The present invention relates to fluid recirculating systems and pertains particularly to an improved hot water recirculating system pumping rate control device.
It has become commonplace in the building industry to use a centrally located hot water heater or boiler circulating system to distribute hot water to a multiple of service locations. This type of hot water system includes the use of a centrally located tank heater, or tube bundle type of heater, with a circulating pump located at the end of the piping for returning water to the heater or boiler. The circulating pump typically operates continuously, or intermittently by way of a timer, to maintain hot water throughout the supply line. The system provides immediate hot water to the multiple hot water service locations. The circulating pump allows the hot water user to have immediate hot water on demand and eliminates the excessive wastage of water and time created by having to wait for the hot water to discharge the standing cooled water and to come all the way from the hot water source.
Over the years, users have discovered problems with the hot water circulating system described above. One particular problem is with the high water flow-rates caused by the circulating pumps not being sufficiently matched to the system. The typical system is provided with a pump that is oversized or too large for the system resulting in high flow rates. High water flow-rates in the circulating system cause excessive wear-and-tear on the plumbing components. These higher water flow-rates also result in energy wastage by way of the elevated temperature of the return line creating excessive heat loss to the atmosphere. The result of the high flow rates is that water is usually returned to the boiler at a temperature only several degrees less than the supply water temperature. The typical supply water is about 130 to 140 degrees and should return to the tank or boiler at about 110 degrees.
One method that has been used in an effort to reduce the problem of wear-and-tear and energy wastage is to use a timer that shuts off the circulating pump during the late evening and early morning. However, this approach does not control the operating temperature in the hot water return line and does not address the problem of excessive flow-rates which cause wear-and-tear damage. The constant cycling of the pump can create a more severe wear-and-tear problem by inducing cyclic loads on the system, and by creating accelerated corrosion of the piping by creating local anode sites due to the thermal expansion and contraction of the metal. Also, the use of timers renders the hot water circulating system inoperable during the late evening and early morning, which can make the hot water users unhappy.
Another approach that has been used to reduce the problem of wear-and-tear and energy wastage is to install a thermostat device at the circulating pump that shuts off the pump when it reaches a certain temperature. Again, this method does not address the problem of excessive flow-rates caused by pump oversizing. Also, the constant cycling of the pump, on-and-off, can create a more severe wear-and-tear condition by causing cyclic loading on the plumbing components, and by creating accelerated corrosion of the piping by creating local anode sites due to the thermal expansion and contraction of the metal.
Many of the above problems can be overcome by using the proper pump size to achieve the proper flow rate in the system. There are basically two methods to determine the proper pump size for a hot water circulating system. The first method involves the designer/engineer to calculate the "head-loss" of the piping circuit. After this is done the designer/engineer then refers to the pump manufacture's "pump performance charts". By comparing the calculated head-loss to the manufacture's pump performance chart, the designer/engineer can specify the proper pump size to achieve the desired flow-rate.
The second method to determine the proper pump size is similar to the first method, however, the approach is slightly reversed. For example, with an existing piping system it is very difficult to calculate the head-loss by determining the length of the pipe and the number and type of fittings. In this case, a flow-rate measuring device is installed to measure the flow-rate produced by a given pump size. With the measured flow-rate and the known pump type, by reference to the pump manufacture's pump performance chart, the head-loss can be determined. With this head-loss data, the properly sized pump could then be specified for the desired flow-rate.
It is therefore desirable that a water recirculation system be available to automatically control the return water temperature at a desired temperature and at the minimum acceptable velocity.